A conventional apparatus for making diffraction-scattering measurements of the particle size distribution of a sample is shown in FIG. 4. As shown therein, a laser beam 1 is projected into a square cylindrical sample cell 2' which contains a sample to be measured.
The sample cell 2' has a square external cross-section and an internal space 2a'. The internal space 2a' of the sample cell 2' is continuously supplied with the sample liquid from an ultrasonic diffusion bath (not shown) by means of a circulating pump (not shown).
Laser beam 1 is transmitted into and through the sample cell 2', and scattered (diffracted) by particulates contained within the sample liquid in the sample cell's internal space 2a'. The scattered light 5 radiating from the sample cell 2' is transmitted through a condenser lens 3, which collects the scattered light 5 and focuses the scattered light 5 upon the detector 4. The detector 4 may be a silicon photodiode.
Thus, the incident laser light 1 is scattered (diffracted) by the particles within the sample liquid, and the scattering or diffraction pattern 5 is directed onto the detector 4 for measurement. In this way it is possible to determine the particle size distribution of the particles within the sample liquid by measuring the intensity of the pattern of the scattered light 5.
A further depiction of the conventional sample cell 2' may be seen in FIGS. 5A and 5B. Therein, a particular light beam 1' from the laser beam 1 is scattered by a particle 6 within sample liquid in the sample cell 2'. The scattered light 5, 5', which is diffracted by the particle 6, is directed towards the rear surface of the sample cell 2b'.
When the particle 6 scatters the light 5 at a small scattering angle .THETA.1, the scattered light 5 is refracted through the sample cell and is allowed to impinge upon the detector 4 by passing through the rear surface 2b' of the sample cell 2'. As an example, when the disbursion medium is water, light which is scattered at a scattering angle .THETA.1 less than approximately 50 degrees will be transmitted through a rear surface 2b', which is normal to the original angle of incidence.
On the other hand, when the particle 6 causes scattered light 5' to have a larger scattering angle .THETA.2, the scattered light 5'interacts with the rear surface of the sample cell 2b' at a minimal angle of incidence .phi.2. Since the angle .phi.2 is minimal, the scattered light 5' is not transmitted through the rear surface 2b', but is reflected from that surface, and is not allowed to reach the detector 4.
Thus, in conventional cells as depicted in FIGS. 4 and 5, when a particle 6 causes light to be scattered at large scattering angles .THETA..sub.2, the scattered light 5' is fully reflected by the surface 2b' without any transmission. This causes the diffraction-scattering measurement to be an erroneous representation of the overall particle size distribution within the sample. In general, where the disbursion medium is water, any particle 6 producing a scattering angle .THETA. greater than or equal to approximately 50 degrees will produce reflected scattered light yielding an erroneous measurement.
The scattering angle is generally inversely proportional to the particle size; the smaller the particle, the larger the scattering angle. Thus, in a conventional apparatus for measuring the distribution of particle size, the particle sizes of particulates having reduced particle diameters have been difficult to measure, and the measurement of the particle size distribution over a wide range has proven to be impossible with a conventional sample cell.